The present invention relates to an axial flow fan, and more particularly to a multiple impeller arrangement with coaxial impellers that rotate in opposite directions. The multiple impeller counter-rotating axial flow fan of the present invention is especially suitable for use in cooling electronic components.
A conventional axial flow fan is generally composed of a driving motor, a cylindrical central hub section mounted on a motor shaft attached to the driving motor, a plurality of blades affixed to the hub, and a housing for encasing the fan or impeller, used herein as equivalent terms. Each of the blades extends radially outward from the central hub section of the fan. The motor shaft is attached to the hub section at a central aperture and thus the hub section may be rotated by the driving motor via the motor shaft. In such an arrangement, the hub section together with the blades are rotated by the motor about an axis of the outer casing in order to force air flow from an inlet area to an outlet area of the fan. The blades of the fan are air foils configured so as to make the blades generate a force in the opposite direction of the blade""s direction of rotation and an air flow that is perpendicular to the blade""s direction of rotation.
Axial flow fans such as Model No. 5920 produced by IMC Magnetics Corporation, the assignee of the present application, are known which utilize a unipolar winding employing a four pole motor where only two of the windings are ON at a time. These fans employ circuitry including circuit elements of a substantial size, such as an inductor to reduce the starting current, transistors large enough to handle the power levels, and large clamping diodes needed to protect the transistors. Such axial flow fans cannot handle input voltages in the range of 57V-64V, are limited to a maximum input voltage of about 56V, and are more typically operated at an input voltage of about 48V.
Model No. 5920 measures two inches in axial width due to both the large size of the diodes, inductors, and transistors used, as well as the number of turns required for a unipolar winding. Furthermore, the axial width of Model No. 5920 is attributed to its 5 blades wherein each blade is characterized by a symmetrical cross-section approximately described as curved flat plates. As such, these blades are not aerodynamically efficient and thus require a larger chord length to meet the performance requirements forcing the dimensions of Model No. 5920 to a two inch axial width.
With the continual increase in the density and load-carrying capability of electronic components on circuit boards, and the consequential increase in heating problems resulting therefrom, axial flow fans are increasingly being used in an effort to combat such heating problems. During the design of such axial flow fans, it is important to make them as small and as cost-effective as possible while maintaining, or even increasing, their ability to cool electronic components. In particular, it is important to reduce the overall size of such a fan as much as possible. For example, the two inches axial width of Model No. 5920 is wider than optimal for use as an axial flow fan for cooling electronic components. Thus, it is desirable to reduce its size while maintaining its performance parameters and design constraints.
One method to reduce the overall size of such a fan is to eliminate large electronic components and reduce the size of other components while maintaining performance parameters and design constraints. For instance, the housing of the axial flow fan may be utilized as a heat sink to reduce the axial width of the fan by eliminating the need for a separate heat sink.
In addition, in order to reduce the overall size of an axial flow fan, it is desirable to utilize narrow chord blades. However, the use of such narrow chord blades results in decreased performance, particularly a decrease in the fan pressure and air flow. These decreases in performance must be offset by varying the design parameters. It is known that, among other factors, the chord length, camber angle, stagger angle, and the cross-sectional shape of the blades are possible factors affecting the performance of the fan. In addition, it is known that by varying the work distribution along a blade""s span, the chord length may be varied along the blade span while maintaining performance parameters.
In theory, the larger the camber angle, the greater the lift force under a constant angle of attack. However, if the camber angle is too large the blade may stall, resulting in a decrease in performance and an increase in noise signature. Consequently, the camber angle must be designed to the proper value.
By way of a further example, a decrease in the work distribution at a radial location will allow for a decrease in chord length with a resultant decrease in velocity exiting the blade at that radial location. Thus, it is desirable to minimize the work distribution at the hub section (root of the blade), since this affects axial width, and to maximize the work distribution at the tip of the blade to deliver the greatest blade exit velocity at the tip. Such an approach was disclosed in U.S. Pat. No. 5,320,493. However, this approach may lead to an intolerable increase in the noise signature of the fan due to the increase in tip velocity exiting the blade as well as an increase in turbulent air exiting the tip of the blades. Thus, it is desirable to locate the maximum work distribution at some favorable location between the root portion and the tip portion.
Furthermore, the cross-sectional shape of the blade affects its velocity distribution. Circular arc profiles, such as NACA series 65 airfoils, exhibit a velocity profile which results in a rapid decrease in the velocity along the suction surface at the trailing edge of the blade. Such a large deceleration gradient results in a more unstable boundary layer, promoting boundary layer separation and hence resulting in loss of lift and greater turbulent air exiting the blade. Thus, the velocity profile of the cross-sectional airfoil must be designed so that a favorable velocity profile is achieved.
Various prior U.S. patents had been developed in this field. For example, U.S. Pat. No. 4,971,520, U.S. Pat. No. 4,569,631, U.S. Pat. No. 5,244,347, U.S. Pat. No. 5,326,225, U.S. Pat. No. 5,513,951, U.S. Pat. No. 5,320,493, U.S. Pat. No. 5,181,830, U.S. Pat. No. 5,273,400, U.S. Pat. No. 2,811,303, and U.S. Pat. No. 5,730,483 disclose axial flow fans. However, the fans disclosed in these patents have not effectively combined the above parameters to overcome the problems described above. In particular no invention discloses a family of airfoil profiles or a blade which delivers the performance of the present invention while reducing the axial width of the fan. Nor does any invention disclose use of such optimized blades in a multiple impeller counter-rotating arrangement to further exploit the reduced width of each impeller individually and to result in a fan having reduced overall size with improved performance.
In the non-analogous field of aircraft rotors, the use of multiple coaxial rotors, as shown in U.S. Pat. No. 3,127,093 to Sudrow, is known. The Sudrow Patent discloses a xe2x80x9cDucted Sustaining Rotor for Aircraftxe2x80x9d which utilizes two sets of coaxial rotors, each of which has a plurality of air foils configured to create lift. These rotors are mounted on motor shafts which are capable of spinning in opposite directions. Such counter-rotating arrangements have been utilized to reduce torque, to reduce axial air flow and to reduce vibration and noise.
Unlike the air foils attached to aircraft rotors, the air foils attached to fan rotors are configured to create air flow. Conventional theory predicts that two identical axial flow fans operating in series in a free flow environment, where there is not substantial downstream flow resistance, will not provide more air flow than one of the axial flow fans operating by itself. Conventional theory also predicts that two identical axial flow fans operating in series in a flow restricted environment, where there is substantial downstream flow resistance, will provide at most twice the air flow of a single fan operating by itself, where the maximum increase is only approached as down stream flow resistance becomes very large. Conventional theory further predicts that placing two otherwise identical axial flow fans in a counter-rotating arrangement, by inverting the rotor of one such fan and rotating said rotor in the opposite direction of the other fan""s rotor, will provide the same amount of air flow as the fans would provide in a co-rotating arrangement. Since using two fans doubles the cost and power requirements of using a single fan, convention theory does not support the use of relatively complicated and bulky counter-rotating arrangements.
U.S. Pat. No. 2,313,413 to Weske discloses an axial flow fan that uses multiple co-rotating impellers with interspersed fixed blades. U.S. Pat. No. 5,931,640 to Van Houten et al. discloses using two counter-rotating fans with oppositely skewed blades for use as vehicle engine cooling fan. These patents disclose that such arrangements allow the fans to develop the required air flows while operating at slower speeds. These patents also teach that the disclosed arrangements reduce parasitic loses and provide improved acoustic properties.
No invention in the prior art discloses a multi-impeller, coaxial, counter-rotating fan that provides increases in airflow compared to a single impeller fan beyond what is predicted by conventional theory. No invention in the prior art discloses a counter-rotating fan that provides more than twice the air flow into a pressurized environment as a co-rotating fan. No invention in the prior art discloses a combination of the factors to formulate a blade which delivers the desired performance while reducing the overall size to that of the present invention. In addition, no invention in the prior art discloses the use of such optimized blades in making dual impeller, coaxial, counter-rotating fan.
Through experimentation, it has been shown that a fan with counter-rotating impellers employing the improved blade design described herein will provide increases in air-flow as compared to a fan with a single impeller that are substantially greater than the increases predicted by conventional theory. In addition, it has been shown that a fan with counter-rotating impellers employing the improved blade design described herein will provide more than twice the air flow into a pressurized environment as an otherwise identical fan with co-rotating impellers. Accordingly, it is an object of this invention to provide a multiple impeller axial flow fan in which the impellers are substantially coaxial and counter-rotating, with substantially improved performance parameters.
It is an additional object of the invention to provide a blade incorporating a family of airfoil sections capable of reducing the axial width of an axial flow fan while maintaining performance parameters and design constraints.
It is yet another object of the invention to provide a blade incorporating a family of airfoil sections which allow for the reduction of the axial width of an axial fan while locating the maximum work distribution between the root portion and the tip portion of the blade.
It is yet another object of the invention to provide a blade incorporating a family of airfoil sections that allow for the reduction of the overall size of an axial fan while maintaining a favorable velocity profile over the suction side of the blade.
It is yet another object of the invention to provide a counter-rotating impeller arrangement that provides an increase in axial air flow which is substantially greater than that predicted using theoretical models.
It is yet another object of the invention to provide a counter-rotating impeller arrangement that provides more than twice the air flow into a pressurized environment as an otherwise identical co-rotating impeller arrangement.
It is yet another object of the invention to exploit the reduction of width of the axial fans by utilizing same to make a multiple, counter-rotating impeller arrangement which provides an increase in axial air flow feasible given certain design constraints of electric fans for use in cooling electronic components.
These and other objects are realized by an axial flow fan structure that includes at least two coaxial rotor assemblies, where each of the rotor assemblies further includes an impeller with a plurality of blades; and where at least one of said rotor assemblies is configured such that it rotates in a direction opposite to the first of said rotor assemblies; and where the blades on each of the rotor assemblies are configured such each impeller forces air in the same axial direction as the other impellers.
These and other objects are further realized by providing said impellers with blades having the following characteristics: a root portion; a tip portion; a leading edge; a trailing edge; the blade having a cross-sectional shape, taken anywhere along a radius of the blade, characterized by a maximum thickness located substantially constantly between about 19% chord to about 20% chord and a maximum camber located substantially constantly between about 45% chord to about 46% chord.
It has been experimentally determined that the acoustic properties of a dual impeller counter-rotating fan can be improved by having a different number of blades on the upstream impeller than are on the downstream impeller. In a preferred embodiment, the upstream impeller consists of thirteen radially extending blades coupled to a circular band and the downstream impeller consists of eleven circumferentially spaced radially extending blades coupled to a circular band.
It has further been determined that air flow is optimized when said multiple counter-rotating impellers are located in a conically shaped hosing. The diameter of the second impeller may be greater than the diameter of the first impeller.